Digestive System: Nutrient Absorption

Human Digestive System
Human Digestive System. Credit: Science Photo Library - PASIEKA/Brand X Pictures/Getty Images

Nutrient Absorption

Have you ever wondered how all of those digested food molecules are absorbed in your body? Well, before those carbohydrates, proteins, and fats can be absorbed, they must be broken down into smaller molecules. Carbohydrate digestion begins in the mouth, protein digestion begins in the stomach, and most fat digestion occurs in the intestines.

Once these biological polymers have been broken down, nutrient absorption can occur.

As food is processed along the digestive tract, most of the nutrients are absorbed through the walls of the small intestines. The nutrients are carried into the bloodstream where they can travel to all parts of the body. Other substances that are absorbed during digestion include vitamins, water, and salt.

Digestive Enzymes

Digestive system glands and organs produce enzymes and acids that are responsible for the break down of food into smaller units. In enzyme naming, the suffix (-ase) is used to signify an enzyme. For example, sucrase is an enzyme that catalyzes the decomposition of sucrose into glucose and fructose. However, not all enzyme names end with the (-ase) suffix.

Mouth: Salivary glands in the mouth secrete several enzymes that facilitate the digestive process. The enzyme amylase aids in the decomposition of carbohydrates into oligosaccharides (small number of monosaccaharide units linked together).

Stomach: Stomach acid and gastric enzymes continue the break down process as the food enters the stomach. The enzyme pepsin degrades proteins into smaller peptides and amino acids.

Small Intestines: Enzymes such as maltase, sucrase, and lactase help to degrade larger carbohydrates into simple sugars.

Digestive juices from the pancreas and liver help to degrade proteins, fats, and nucleic acids.

Pancreas: Pancreatic juices contain amylase, lipases, proteases, and nucleases. Lipases decompose lipids into fatty acids and glycerol. Proteases degrade proteins and nucleases degrade DNA and RNA into their nucleotide subunits. The pancreas delivers these digestive enzymes to the small intestine through ducts.

Liver: The liver produces bile, which is stored in the gall bladder. Bile fluid contains bile acids, which dissolve fats. Bile is secreted into the small intestines through bile ducts.

Nutrient Absorption

Nutrients absorbed by the small intestines are delivered to various parts of the body through the circulatory system and the lymphatic system. Before gaining access to blood or lymph, the nutrients must pass through the epithelium of the intestines. Some substances, such as vitamin B12, are transported across the intestinal membrane by facilitated diffusion. Other substances including glucose, amino acids, and iron are actively transported across the membrane.